Discord: add trusted channel topics on new sessions

src/auto-reply/templating.ts

@@ -98,6 +98,8 @@ export type MsgContext = {
   GroupSubject?: string;
   /** Human label for channel-like group conversations (e.g. #general, #support). */
   GroupChannel?: string;
+  /** Channel topic/description (trusted metadata for new session context). */
+  ChannelTopic?: string;
   GroupSpace?: string;
   GroupMembers?: string;
   GroupSystemPrompt?: string;